We always knew that trees are good air filters, but it's been hard to quantify just how much. One new study from Lancaster University, in the U.K., tried to do so. What they found is quite interesting, and one more reason to hug a tree today!

Map of global air pollution compiled by the World Health Organisation/Public Domain

The scientists started by measuring how much air pollution go into a certain number of houses in Lancaster using dust monitoring devices and by swiping surfaces and then analyzing what was collected with magnetic remanence, a technique that provides information on concentrations of iron-bearing particles.

Then the team placed a screen of 30 young silver birch trees in wooden planters in front of four of the houses, including one of the control houses, for 13 days. Wipes from all eight houses showed that ones with the tree screens had 52 to 65% lower concentrations of metallic particles. A comparison of all of the dust monitoring data from the two original control houses confirmed that drop, showing a 50% reduction in PM1, PM2.5, and PM10 in the house with the trees in front.

By examining the silver birch leaves with a scanning electron microscope, the researchers confirmed that the hairy surfaces of the leaves trapped metallic particles. Like the particles measured inside the houses, these metallic particles are most likely the product of combustion and brake wear from vehicles passing by. Previous work has indicated a strong correlation between the amount of material identified by magnetic remanence and benzo(a)pyrene, a carcinogenic polycyclic aromatic hydrocarbon found in particulates, Maher says. (source)